Simulations Reveal Surprising News
about Black HolesComputer reveals that life around black holes is
turbulent and violent

For more than 30 years, astrophysicists have believed
that black holes can swallow nearby matter and release a
tremendous amount of energy as a result. Until recently,
however, the mechanisms that bring matter close to black
holes have been poorly understood, leaving researchers
puzzled about many of the details of the process.

Now, however, computer simulations of black holes
developed by researchers, including two at The Johns
Hopkins University, are answering some of those questions
and challenging many commonly held assumptions about the
nature of this enigmatic phenomenon.

"Only recently have members of the research team
— John Hawley and Jean-Pierre De Villiers, both of
the University of Virginia — created a computer
program powerful enough to track all the elements of
accretion onto black holes, from turbulence and magnetic
fields to relativistic gravity," said
Julian Krolik(pictured at right), a
professor in the Henry
A. Rowland Department of Physics and Astronomy at Johns
Hopkins, and co-leader of the research team. "These
programs are opening a new window on the complicated story
of how matter falls into black holes, revealing for the
first time how tangled magnetic fields and Einsteinian
gravity combine to squeeze out a last burst of energy from
matter doomed to infinite imprisonment in a black hole."

Close to the black hole's outer edge, where the
Newtonian description of gravity breaks down, ordinary
orbits are no longer possible. At that point — or so
it has been imagined for the past three decades —
matter plunges quickly, smoothly and quietly into the black
hole. In the end, according to the prevailing picture, the
black hole — except for exerting its gravitational
pull — is a passive recipient of mass donations.

The team's first realistic calculations of matter
falling into black holes has strongly contradicted many of
these expectations. They show, for instance, that life in
the vicinity of a black hole is anything but calm and
quiet. Instead, the relativistic effects that force matter
to plunge inward magnify random motions within the fluid to
create violent disturbances in density, velocity and
magnetic field strength, driving waves of matter and
magnetic field to and fro. This violence can have
observable consequences, according to research team co-
leader Hawley.

"Just like any fluid that has been stirred into
turbulence, matter immediately outside the edge of the
black hole is heated. This extra heat makes additional
light that astronomers on Earth can see," said Hawley. "One
of the hallmarks of black holes is that their light output
varies.

Although this has been known for more than 30 years, it has
not been possible to study the origins of these variations
until now. The violent variations in heating — now
seen to be a natural byproduct of magnetic forces near the
black hole — offer a natural explanation for black
holes' ever-changing brightness."

One of the most striking properties of a black hole
is its ability to expel jets at close to the speed of
light. While it has long been expected that magnetic fields
are crucial to this process, the latest simulations show
for the first time how a field can be expelled from the
accreting gas to create such a jet.

Perhaps the most surprising result of the team's new
computer simulations is that the magnetic fields brought
near a rotating black hole also couple the hole's spin to
matter orbiting farther out, in the same way that a car's
transmission connects its rotating motor to the axle. Says
Krolik, "If a black hole is born spinning extremely
rapidly, its 'drive train' can be so powerful that its
capture of additional mass causes its rotation to slow
down. Accretion of mass would then act as a 'governor,'
enforcing a cosmic speed limit on black hole spins."

According to Krolik, that "governor" may have strong
implications for many of the most striking properties of
black holes. It is widely thought, for example, that the
strength of a black hole's jet is related to its spin, so a
"spin speed limit" might determine a characteristic
strength for the jets, Krolik said.

Funded by the National Science Foundation, this
research is being published in a series of four papers in
The Astrophysical Journal. ((De Villiers et al 2003,
ApJ 599, 1238; Hirose et al. 2004, ApJ 606, 1083; De
Villiers et al. ApJ 620, 879; Krolik et al. April 2005
ApJ in press.)) The simulations were performed at the
NSF-supported San Diego Supercomputer Center.
The research team also included Shigenobu Hirose, also
of Johns Hopkins.

Color photos of Dr. Krolik are available upon request.
Contact Lisa De Nike.

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